This invention relates to the use of water-atomized powdered iron in a magnetorheological (MR) fluid.
Magnetorheological (MR) fluids are substances that exhibit an ability to change their flow characteristics by several orders of magnitude and in times on the order of milliseconds under the influence of an applied magnetic field. These induced rheological changes are completely reversible. The utility of these materials is that suitably configured electromechanical actuators that use magnetorheological fluids can act as a rapidly responding active interface between computer-based sensing or controls and a desired mechanical output. With respect to automotive applications, such materials are seen as a useful working media in shock absorbers, brakes for controllable suspension systems, vibration dampers in controllable power train and engine mounts and in numerous electronically controlled force/torque transfer (clutch) devices.
MR fluids are non-colloidal suspensions of finely divided (typically one to 100 micron diameter) low coercivity, magnetizable solids such as iron, nickel, cobalt, and their magnetic alloys dispersed in a base carrier liquid such as a mineral oil, synthetic hydrocarbon, water, silicone oil, esterified fatty acid or other suitable organic liquid. MR fluids have an acceptably low viscosity in the absence of a magnetic field but display large increases in their dynamic yield stress when they are subjected to a magnetic field of, e.g., about one Tesla. The iron particles are kept suspended in the liquid by the action of a thixotrope or anti-settling agent. Special additives are also used to reduce oxidation of the base fluid and iron particles, reduce friction, reduce wear, and improve durability.
In the context of automotive applications, MR fluids have been developed to pass shock absorber durability testing, while minimizing settling and in-use thickening. This has largely been accomplished by careful specification of components of the formulation. For example, prior art fluids have used soft carbonyl iron to provide low-coercivity, high yield strength, and consistent properties over life for MR fluids, together with a single-component organoclay thixotrope to minimize settling of the MR fluid. By way of example, U.S. Pat. No. 6,203,717 describes the use of single-component organoclays to achieve stable MR fluids. In addition, typical prior art MR fluids contain additives, such as organomolybdenums, ZDDP, thiocarbamates, and phosphorous-containing compounds in low concentration (about 1-2%) to minimize in-use thickening and reduce wear of mechanical components.
First generation MR fluids generally have an operating temperature range of xe2x88x9240-70xc2x0 C., with excursions up to 105xc2x0 C. These fluids pass the 1 Million cycle side-loaded durability tests with no in-use thickening, and exhibit acceptable settling properties. Although prior art first generation MR fluids do have the desirable properties listed above, it has been found that compositions formulated in accordance with the prior art still tend to have the following disadvantages, which are serious impediments to their utilization in future applications, including second generation and third generation automotive applications: high cost, due to expensive soft carbonyl iron powder ($6.15-$18 per pound); high volatility, particularly when the base fluids are chosen to provide acceptable low-temperature viscosity, or alternatively, unacceptably high viscosity when the base fluids are chosen to provide high volatility at high temperature; and noticeable loss of performance over time due to oxidation of the soft carbonyl iron particles. The second and third generation MR fluids require a wider operating temperature range, specifically xe2x88x9240xc2x0 C.-130xc2x0 C. continuous exposure, with up to 150xc2x0 C. excursions. The second generation MR fluids will require 25% higher on-state forces, and third generation MR fluids will require 100% higher on-state forces. Both second and third generation MR fluids will require a 25% decrease in off-state forces and 50% decrease in cost, as well as lower friction. These requirements must be met without any compromise in durability and settling performance.
The problems of prior art MR fluids can be traced to specific material components and combinations, rather than inherent shortcomings of the technology itself. For example, viscosity and volatility in prior art fluids can be attributed to the combinations of base fluid, single-component organoclay thickener, and additives that are taught in the prior art. The combination of the single-component clay thickener with the additives generally results in fluids that have high viscosity, when the fluid is formulated for optimum stability to settling. Additionally, the use of soft or reduced carbonyl iron as taught in the prior art is the most significant factor in the cost of the raw materials that make up the MR fluid. In a 20 vol. % iron MR fluid, for example, the majority ( greater than 90%) of the raw material cost comes from the carbonyl iron powder. Also, carbonyl iron powder is  greater than 99% pure iron, and is therefore highly susceptible to oxidation. It is likely that oxidation of the iron powder is the cause for observed loss in magnetic performance in MR dampers over time.
The carbonyl process involves the thermal decomposition of iron pentacarbonyl that yields high purity iron. The particles are smooth and generally spherical, with diameters typically in the range of 1-10 xcexcm. However, carbonyl iron is liable to oxidize in use, in part due to its high level of purity. Oxidation of the carbonyl iron has been observed in MR fluids used in fan clutch and shock absorber applications, for example. Oxidation can occur as a result of exposure to high temperatures and/or moisture. Carbonyl iron powders typically begin to oxidize in air at temperatures well below 200xc2x0 C. In a clutch application, for example, the MR fluid often reaches over 200xc2x0 C. Oxidation of the iron particles can reduce the magnetorheological effect of the fluid by as much as 20% or more. Iron oxide exhibits poorer magnetic properties than pure carbonyl iron. Moreover, the yield stress for the MR fluid decreases over time, and this is believed to be a result of one or both of the oxidation of the carbonyl iron particles or a change in the shape and size distribution of the particles. This reduction in effectiveness can severely affect device performance.
There is thus a need for an MR fluid formulation that maintains the durability, anti-settling and thickening properties of prior MR fluids while improving oxidation resistance and magnetic performance and reducing cost.
The present invention provides an MR fluid formulation comprising water-atomized iron powder dispersed in a liquid vehicle, wherein the atomized iron powder contributes to a higher magnetic effect, a lower viscosity and suitability for high temperature applications due to a low oxidation rate, these benefits particularly noticeable in comparison to similar fluids using carbonyl iron particles. In an exemplary embodiment of the present invention, the magnetizable particles are prepared by controlled water atomization and comprise iron having a passivating oxide surface layer, and a mean diameter in the range of about 8-25 xcexcm. In a further exemplary embodiment, the passivating oxide layer is an alloy of iron and at least one other metal and their oxides. In another exemplary embodiment of the present invention, the liquid vehicle is a mixture comprising at least two liquid components of different surface functionality. In this embodiment, an organoclay stabilization mixture is added in which at least one organoclay is selected for each liquid vehicle component with each organoclay having a surface chemistry that renders it preferentially compatible with the surface functionality of one of the liquid components relative to its compatibility to the remaining liquid components whereby it is effective to stabilize, or gel, that component. The use of the dual-clay mixture in combination with standard additives provides stable, durable MR fluids that have low viscosity. Exemplary MR fluid formulations of the present invention utilize a high-viscosity, low volatility base fluid, water-atomized iron powder, multi-component organoclays and multi-component additives to achieve the desired viscosity and durability of a fully formulated MR fluid which will satisfy the requirements of second and third generation MR fluids.
The present invention further provides a method of making an MR fluid. The method includes blending a liquid vehicle mixture of at least two liquid components, each having a different surface functionality and adding at least one surface-treated organoclay for each liquid component to the liquid vehicle mixture. The surface treatment of each organoclay renders that organoclay preferentially compatible with the surface functionality of its respective liquid component relative to the compatibility of the organoclay to the remaining liquid components. The method further includes dispersing magnetizable particles in the liquid vehicle mixture, which particles have been prepared by controlled water atomization and comprise iron having a passivating oxide surface layer and a mean diameter in the range of about 8-25 xcexcm. The method may further include adding one or more additives to the liquid vehicle mixture, such as an organomolybdenum complex, an organomolybdenum thiocarbamate, a zinc dithiophosphate, and an organothiocarbamate.